Transfer switch vs interlock kit vs power inlet
A transfer switch and an interlock kit both let a generator feed your house wiring without sending power back to the utility line. A power inlet is only the plug on the outside wall, and an automatic transfer switch does the changeover without you.
Why they exist: backfeed
Your house is connected to the utility. If a generator feeds the house wiring while the main breaker is on, power flows backward through the meter and the transformer. A transformer turns that low voltage into a much higher one on the street. This is called backfeed. It can electrocute line workers who think the line is dead, and it can damage the generator when the grid returns.
Every device below solves the same problem: the house is connected to the utility or to the generator, never both.
The four parts
| Part | What it does | Who operates it |
|---|---|---|
| Manual transfer switch | A small sub-panel. You move selected circuits (fridge, furnace fan, lights) from utility to generator. | You |
| Interlock kit | A metal plate on the main panel. It blocks the generator breaker and the main breaker from being on together. | You |
| Automatic transfer switch (ATS) | Senses the outage, starts the generator and changes over. Used with standby generators. | Automatic |
| Power inlet | A weatherproof receptacle on the outside wall. The generator plugs in with a cord. It needs a switch or interlock behind it. | You, plug only |
How to choose
Manual transfer switch. You choose which circuits are covered, so a smaller generator can run the load. You must decide the circuit list before the outage.
Interlock kit. It uses the existing panel, so the whole panel is available. The load is then your responsibility: with the interlock, you turn off big loads by hand, because a small generator cannot run everything. The kit must match your panel make and model.
Automatic transfer switch. It pairs with a standby generator. Power returns in seconds without you at home. It is installed by a licensed professional with a gas or fuel connection.
Battery systems. Many home batteries include their own transfer hardware. Portable power stations usually do not, and you plug appliances into them directly. Use a power station for a manual transfer switch only if its manual says it supports that.
Permits, electricians and your utility
Work inside the service panel carries shock and fire risk, and most places require a permit and inspection. We do not quote code articles here. Rules differ by state, county and utility, and they are revised on a cycle. Check your local code and your utility. Ask the electrician which edition applies, and whether an inspection is required before you use the system.
You can do these things yourself: choose a generator, measure its output, decide which circuits you need, and make a list. Leave panel work to a licensed electrician.
Wire size and inlet rating
Watts divided by volts gives amps. A typical median portable generator in our catalogue is rated at 4,000 W, which is 16.7 A at 240 V. The median standby generator is rated at 22,000 W, which is 91.7 A at 240 V. An inlet rated at 30 A at 240 V can carry 7,200 W, and one rated at 50 A can carry 12,000 W. Match the inlet, cord, breaker and in-wall wire to the generator output.
Long runs lose voltage. Use the wire size calculator for the cable length and amps. The result is guidance; the electrician follows local code.
Before the next outage
Decide your loads with the backup power calculator. Compare the options in generator vs battery backup. If a pump is on your list, read well pump backup power. See inverter generators for output ratings. Test the setup with the utility on, never for the first time in an outage. If your generator shows a fault, check error codes.
FAQ
Can I plug a generator into a wall outlet?
No. A cord with two male ends can energize the house wiring and the utility lines outside. Line workers and neighbors can be killed. Use a transfer switch or interlock installed to your local code.
What is the difference between a transfer switch and an interlock kit?
A transfer switch is a separate panel that moves chosen circuits between utility and generator. An interlock is a mechanical plate on your main panel that stops the main breaker and the generator breaker from being on together. Both prevent backfeed.
Do I need a permit and an electrician?
In most places, yes. Work inside the service panel is usually permitted and inspected, and the utility may need to be told. Check your local code and your utility before buying anything.
Does an inlet replace a transfer switch?
No. A power inlet is only the plug on the outside wall. It still needs a transfer switch or interlock behind it.
General information, not electrical advice. Figures come from published product data and Ohm's law (amps = watts ÷ volts).